In wireless OFDM communication systems, a single OFDM symbol is composed of multiple subcarriers in frequency. Data modulation symbols are directly mapped onto these subcarriers. Some of the subcarriers may be reserved for reference/pilot symbols to assist demodulation at the User Equipment (UE). Further, all available subcarriers may be sub-divided into sets or groups of subcarriers for allocation to users with reduced overhead of signaling.
In typical OFDM based systems like 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE), a block of 14 consecutive OFDM symbols are referred to as a subframe. Each sub-carrier location in each of the OFDM symbols is referred to as a resource element (RE), since a single data modulation symbol can be mapped to such a resource element. A resource block (RB) is defined as a block of REs composing of a set of 12 consecutive sub-carrier locations in frequency and 7 symbols of a slot. Each subframe is made of two slots, and hence 14 symbols. A minimum resource unit allocated to a user is the two RBs corresponding to two slots in a subframe for a total of 2×12×7 REs.
Some of the REs in the RB maybe reserved for Control Channel functions, the locations of which is known to the UE. The disclosure more specifically pertains to the data carrying portion of the RB. This is, for example, referred to as Physical Data Shared Channel (PDSCH) in Release-8 LTE. REs in the rest of the document refer to REs in such data carrying portion of the RBs.
Some of the REs in a RB are reserved for reference symbols (RSs) (also referred to as pilots) to help in the demodulation and other measurements at the UE. These reference symbols, as defined in Release 8 LTE can be further divided into two types. The first type is cell-specific reference symbols (CRS), which are cell-specific and “common” to all users, and are transmitted in all the RBs. CRS may or may not correspond to actual physical antennas of the transmitter, but CRS are associated with one or more antenna “ports”, either physical or virtual.
The second type is user-specific or dedicated reference symbols (DRS), which are user-specific and hence applicable to that user only, and allocated in the RB's allocated to that user. Furthermore, DRS typically correspond to “precoded” or beam-formed RS, which can be directly used by a user for the demodulation of the data streams.
The location of the reference symbols is known to the user equipment from higher layer configurations. For example, depending on the number of antenna ports as configured by a transmission unit, a user equipment knows the location of all the reference symbols corresponding to all configured antenna ports. As another example, when a user equipment is instructed to use DRS, the user also knows the DRS locations which may depend on the user identification.
The data symbols intended for a user in his allocated RBs are mapped to the remaining set of REs after provisioning for the reference symbols. There is no ambiguity on data mapping between the user equipment and the transmission unit once the RS locations are clear.
In a future migration of a system, user-specific RS may be used widely with advanced Multiple-Input Multiple-Output (MIMO) modes like Coordinated Multipoint transmission (CoMP) and multi-user (MU) MIMO modes. Multiuser MIMO schemes refer to MIMO schemes where data is transmitted simultaneously to more than one user from the same set of RBs. A coordinated multipoint scheme is a scheme where data is transmitted to one or more users by coordinated scheduling and/or joint transmission from one or more transmission points. It is clear in such a case, that a user allocation may have to support reference symbols that may correspond to other users and/or other transmission points.
On the other hand, the advantage of using DRS for demodulation at the user equipment has two primary advantages. The actual transmission mode details, such as number of users, number and identity of transmission points, etc, need not be signaled to a user, as long as he can reconstruct the channel based on the DRS. Further, this allows more dynamic changes to the transmission mode(s) without the need to semi-static configuration by the higher layers, since a user need not be made aware of such configurations explicitly.
However, due to obligation of the assisting transmission points in a CoMP transmission or provisioning reference symbols for other users in a MU transmission, additional reference symbols may have to be supported. There is a need for a method of signaling a particular type of resource element in a wireless communication system.
The various aspects, features and advantages of the invention will become more fully apparent to those having ordinary skill in the art upon a careful consideration of the following Detailed Description thereof with the accompanying drawings described below. The drawings may have been simplified for clarity and are not necessarily drawn to scale.